OSL Script from External Source?
Select the External mode on the script node and set the filepath to your .osl source file or a compiled .oso file. Or, if you put your shaders under the shader search path (see below), you can simply specify the filename.



From the documentation:


  The External mode can be used to specify a .osl file from a drive, and
  this will then be automatically compiled into a .oso file in the same
  directory. It is also possible to specify a path to a .oso file, which
  will then be used directly, with compilation done manually by the
  user. The third option is to specify just the module name, which will
  be looked up in the shader search path.
  
  The shader search path is located in the same place as the scripts or
  configuration path, under:
  
  Linux

 $HOME/.config/blender/2.79/shaders/

  
  MS-Windows

C:\Users\$user\AppData\Roaming\Blender Foundation\Blender\2.79\shaders\

  
  macOS

/Users/$USER/Library/Application Support/Blender/2.79/shaders/